Among cathode materials for secondary lithium batteries, elemental sulfur has a very high theoretical capacity, 1672 mAhg−1 against lithium, which is considerably greater than that of many commercially used transition metal phosphates and transition metal oxides. In addition, elemental sulfur also provides several other advantages as a cathode material for a secondary lithium battery, including in particular a low cost and a widespread availability. Sulfur has consequently been studied extensively as a cathode material for secondary lithium batteries and is considered a promising candidate for a cathode material for secondary lithium batteries that may be used in electric and hybrid electric vehicles.
Despite this promise, implementation of Li—S secondary battery systems for high power applications has been problematic for various reasons. For one, sulfur by itself has relatively low electrical conductivity. Thus, desirable are methods and materials that provide an opportunity to fully realize the advantages of sulfur as a cathode material within a Li—S secondary battery system.
Carbon, from sources such as coal, can be used to provide conductivity to materials, and has been used in lithium ion electrodes for this purpose.
While lithium sulfur (Li—S) cathode material has long enjoyed a significant (10×) specific capacity advantage over current lithium-ion batteries, Li—S chemistries have been impractical due to poor cycle life and a high rate of discharge. The polysulfide shuttling reaction between sulfur and its lithiated compounds has limited the development of batteries based on the Li—S chemistry because the reaction leads to irreversible material losses in the battery that reduces energy storage capacity over time. Shuttling is a cyclic process in which long-chain lithium polysulfides, (Li2S., 2<n<8), generated at the cathode during charging, dissolve into the electrolyte and migrate to the anode by diffusion where they react with the lithium electrode in a parasitic fashion to generate lower-order polysulfides, which diffuse back to the sulfur cathode and regenerate the higher forms of polysulfide. Since this polysulfide shuttling or dissolution takes place at the expense of the available electroactive sulfur species, the reversibility of sulfur and/or sulfide is broadly considered a major technical barrier towards commercialization of high-energy Li—S batteries. Another limitation is elemental sulfur is a poor electrical conductor (with a conductivity z 5×10−30 S cm−1 at 25° C.), which has limited the rate at which a conventional Li—S battery can be discharged/charged.
Thus, there remains a need for sulfur-containing cathode materials for lithium secondary cell with improved conductivity and cycle life.